The present invention relates to signed point encoders used in the transmission of digital information over an analog medium connected to a digital network, particularly in the context of Pulse Code Modulation (PCM) modems.
The world based on the Internet has seen tremendous growth in recent months. As more users begin browsing and downloading information from the World Wide Web, there has been a great desire to increase the data transmission rate, or simply called data rate. The desire is even greater for users accessing the Internet through an Internet service provider (ISP), since most users are linked up to the "Net" through a personal computer and a modem. Conventional analog modems, such as V.34 modems, however, view the public switched telephone network ("PSTN") as an analog channel, even though the signals are digitized for communications throughout most of the PSTN. As such, various effects of and impairments due to signal quantization impose a limitation on the data rate of the channel to about 35 Kbps. This limit has been commonly known as Shannon's Limit. (See C. E. Shannon and W. Weaver, The Mathematical Theory of Communication, University of Illinois Press, 1949).
There has been much recent development of high-speed communications technology based on PCM modems, where data rates of at least 56 Kbps are said to be actually attainable. The PCM modem technology is based on the simple realization that the PSTN is increasingly a digital network and not an analog network. Also, more and more central site modems are connected to the PSTN through digital connections, i.e., T1 in the U.S. and E1in Europe, without requiring a CODEC (coder/decoder). A CODEC is a device which connects the digital portion of the network to the analog local loop and converts between analog and digital.
The conventional modem, however, still interprets this digital stream as the representation of the modem's analog signal. With the PCM modems, however, a much higher data rate can be achieved without the complicated task of re-wiring the user's site or modifying the telephone network. It should be recognized that "central site" modems refer to those modems installed at an ISP, or at a corporation, for example, to allow many simultaneous connections for remote local area network (LAN) access.
The recent 56 Kbps technology seeks to address an impaired section of the communications path of the PSTN digital network, where the impairment is due to the hybrid and the copper wire interface between the telephone central office and the user's home, usually referred to as the analog local loop.
Since recently, much has been described about PCM modems and how they can and should facilitate downstream data communication at a much higher rate than the present paradigm. For example, the PCM modem has been the subject of a recent Telecommunications Industry Association (TIA) Technical Committee TR-30 Standards meeting on Oct. 16-17, 1996. The submitted technical contributions include Guozhu Long's DC Suppressor for 56K Modems, Guozhu Long's Two-Step Mappingfor 56K Modems, David C. Rife's 56 Kbps Channels, Veda Krishnan's V.pcm Modem Standard, Vedat Eyuboglu's PCM Modems: A Technical Overview, Richard Stuart's Proposal for a High Speed Network Access Modem, and Vladimir Parizhsky's U.S. Robotics'x2 Technology: Technical Brief. These contributions are hereby incorporated by reference.
Also, there have been recent publications on the overall data communication system based on the PCM modem. The first one is a 1995 presentation disclosed by Pierre A. Humblet and Markos G. Troulis at Institute Eurecom, entitled The Information Driveway, 1995, which purports to explain the basic concepts of the high speed modem. The second one is a PCT Patent Publication, dated Jun. 13, 1996, International Publication Number WO/9618261, by Brent Townshend, which discloses a High Speed Communications Systems for Analog Subscriber Connections. This Publication, on pages 17-19, discloses an overall high speed system based on PCM modems (which also implements DC null elimination on the transmitter side). These papers provide a fair reference to the basics of the high speed PCM modems and their environment, and are hereby incorporated by reference.
Additionally, U.S. Pat. No. 5,528,625, issued to Ender Ayanoglu of AT&T, dated Jun. 18, 1996, entitled High Speed Quantization-Level-Sampling Modem With Equalization Arrangement, discloses a QLS modem for high-speed data communication. Another U.S. patent also issued to Ender Ayanoglu of AT&T, U.S. Pat. No. 5,394,437, dated Feb. 28, 1995 entitled High-Speed Modem Synchronized To A Remote CODEC, discloses a high-speed modem for data transmission over an analog medium in tandem with a digital network. These references are also hereby incorporated by reference.
FIG. 1 depicts a conceptual diagram of the typical high-speed communication path using PCM modem technology. An ISP, or central site, 100 is digitally connected to a telephone network 130 through its transmitter 110 and receiver 120. The network 130 is connected to a local loop 150 through a central office line card 140. The line card typically has a PCM CODEC implemented therein. The local loop 150 is connected to the user's PC at the user's site through the user's modem 160. As can be appreciated by those skilled in the art, the connection between the ISP modem transmitter 110 to the telephone network 130 is a digital connection with a typical data rate of about 64 Kbps. Since the parameters of the telephone network 130 and line card 140 are dictated and set by the telephone company's specifications and operation (and particularly their use of the .mu.-law signal point constellation), the central site transmitter 110 needs to transmit the digital data in a particular way to fully exploit its digital connection to the network. However, dealing with .mu.-law constellations, shell mapping, and PCM-based modem systems in this new paradigm has some obstacles.
For example, in the V.34 paradigm, the shell mapping algorithm is essentially designed to select ring indices in a manner which minimizes average transmission power based on, inter alia, the assumption that the average power of each ring is approximately proportional to its ring index, and based on the further assumption that any particular constellation can be scaled to meet the transmit power level requirement. In a PCM modem context, on the other hand, the signal points are selected from a fixed, non-uniformly spaced set of levels determined by the PCM codec in accordance with the well-known .mu.-law algorithm. Hence, the above assumptions made for the V.34 shell mapping algorithm break down in the context of PCM modems. Furthermore, in order to obtain optimum performance using known shell mapping techniques, an entirely new cost function different from the cost function employed in V.34 would have to be defined, and a new mapping algorithm constructed for use in a PCM modem context. The implementation of such a new cost function and mapping algorithm would not significantly exploit the V.34 algorithm, which is currently utilized by a substantial number of modems currently in use. In the V.34 signal-point encoding model, typically employed by a transmitting modem at an internet service provider (ISP server), the encoder function is often divided into two realms, including a coding part and a mapping (or shaping) component. The coding component often involves error-correction coding, whereas the mapping component strives to minimize the transmission power in view of the restraints imposed by the coding process. For example, the traditional V.34 coding function involves the use of convolutional trellis codes, whereas the mapping is in the form of shell mapping.
The shell mapping algorithm employed in V.34 is one of the more complex functions in a V.34 modem. For a more complete description of the V.34 Recommendation, see ITU-T Recommendation V.34, published September, 1994 by the International Telecommunication Union, the entire contents of which is hereby incorporated by this reference. Essentially, the V.34 encoding algorithm takes a block of bits corresponding to a mapping frame of eight (8) symbols, and maps a part of that block to a set of eight (8) ring indices, which are used to determine a subset of the constellation from which the transmitted signal points are selected. In this context, the subsets are, as the name indicates, in the form of concentric rings around the origin. As such, the energy of the signal points in a given ring is within a certain range, which energy range increases with increasing distance from the origin. Thus, the index of the ring is a fairly accurate approximation of the contribution to signal power of a point in that ring. The V.34 shell mapping algorithm uses this simple relationship to select sets of ring indices where the sum of the indices is the smallest. Sets of ring indices with higher sums tend to be omitted, thus optimizing transmit power. As a result, in the V.34 shell mapping algorithm, the innermost rings are selected most often, and the outermost rings are selected least often.
In PCM modems, however, the signal points are selected from a non-uniform set of levels determined by the .mu.-law algorithm. Many of the characteristics of the V.34 constellation are therefore lost in a PCM modem context, for example the linear relationship between ring index and that ring's contribution to transmission power.
U.S. Pat. No. 5,428,641, issued Jun. 27, 1995 to Long, and U.S. Pat. No. 5,465,273, issued Nov. 7, 1995 to Cole, generally disclose shell and frame mapping techniques that may be used in conjunction with modems and, more specifically, with V.34 transmission protocols. Both of these patents are hereby incorporated by reference. Guozhu Long's contribution to the TR-30 Standards Meeting, entitled Two-Step Mapping for 56K Modems, discloses a shell mapping algorithm intended to replace the standard V.34 mapping algorithm. Long's mapping technique is designed to reduce the error rate associated with 56K modems. However, such use of a new mapping algorithm may be impractical to implement or undesirable in light of the widespread use of the V.34 mapping algorithm.
A technique is therefore needed which overcomes the shortcomings of the prior art. In particular, a long felt need exists for a PCM-based signal point encoding methodology which conforms to the transmission power limitations imposed by the Public Switched Telephone Network (PSTN), which facilitates the minimization of transmission errors, and which exploits many of the advantageous features of the V.34 shell mapping algorithm.